As mobile communication services become increasingly diversified, for example, applications of social networks and e-payment using in radio communication systems become more and more popular, demands for service transmission between adjacent users increase continuously. With a capability of reducing a power consumption of a user terminal and improving spectrum efficiency, Device-to-Device (D2D) communication for adjacent users is getting more and more attention. FIG. 1 is a schematic diagram illustrating the D2D communication between two user equipments. As shown in FIG. 1, the D2D communication, which is also called Proximity Service (ProSe), refers to a direct transmission of service data from a source user equipment (UE) to a target UE via an air interface, without forwarding by a base station or a core network. For near field communication users, D2D can not only save radio spectrum resources but also reduce the data transmission workload of the core network.
In cellular communications, when two UEs are communicating with each other, generally, each UE is incapable of sensing a location of the opposite UE by itself, therefore, a connection is established between the two UEs through a network-side device (e.g. a base station or a device of a core network). For the D2D communication, a premise of establishment of a communication link lies in a mutual discovery of UEs. Because D2D communication has a half-duplex characteristic which is reflected in discovery as follows: a UE is incapable of receiving a discovery signal after sending the discovery signal or sending a discovery signal after receiving the discovery signal. Thus, when two UEs respectively send the discovery signal at the same time, the UEs cannot be discovered by each other, which limits a application of D2D communication.
Thus, because of a half-duplex characteristic of the D2D communication which disenables UEs synchronously sending the discovery signal to be discovered by each other, D2D communication is limited in application.